FIG. 5 is a block diagram of a conventional digital broadcast receiving apparatus in mobile terminals. FIGS. 6A and 6B illustrate a relation between a time division multiplexed program and power supply controlling timing in a conventional digital broadcast receiving apparatus.
A conventional digital broadcast receiving apparatus includes antenna portion 1602, receiving circuit 1142, power supplying circuit 1143 and demodulating portion 1141.
Antenna portion 1602 receives at antenna 1601 a radio wave in a space to convert the radio wave into an RF receiving circuit input signal at BPF (a bandpass filter) 1104.
Receiving circuit 1142 consists of channel-selecting portion 1109 and other portions. Channel-selecting portion 1109 includes VCO (voltage controlled oscillator) 1106, synthesizer 1107 and controlling voltage generating circuit 1108. Portions of receiving circuit 1142 other than channel-selecting portion 1109 include RF variable gain amplifier 1110, frequency converting circuits 1111 and 1113, 90° phase shift circuit 1112, low pass filters 1114 and 1115, frequency adjusting circuit 116 and base band variable gain amplifiers 1117 and 1118. Receiving circuit 1142 selects a desired channel signal from inputted signals and converts the selected signal into a base band signal.
Demodulating portion 141 demodulates the base band signal to output a transport stream (referred to as a TS, hereinafter).
Power supplying circuit portion 1143 supplies receiving circuit 1142 and demodulating portion 1141 with power.
FIG. 6A shows time division multiplexing of digital broadcast transmitted after time division multiplexing. In FIG. 6A, signals from Program 1 to Program 4 are time multiplexed.
Signal information recover circuit 126 of demodulating portion 1141 obtains program timing from Program 1 to Program 4 on the basis of the TS. In the case that Program 3 is desired, for example, a timing signal shown in FIG. 6B is outputted. Power supply on/off timing generating circuit 1134 uses the timing signal to generate a signal whose voltage level is capable of on/off control of power supplying circuit 1137 of receiving circuit 1142 other than channel-selecting portion 1109. As described above, turning off during an undesired program of the TS a supplying power source to receiving circuit 1142 other than channel-selecting portion 1109 in which it takes long time to become stable after turning on the power supply allows a low consumption power to be enabled and battery consumption to be restrained.
As information of former art references concerning to the invention, US Laid-Open Patent Application No. 2002-015941 A1 is known, for example.
The digital broadcast receiving apparatus to be used, which is built in a mobile terminal, is, however, used as a mobile, so that a receiving point is changed. Accordingly, a state of a radio wave changes as time passes, and therefore, a stable reception is impossible. Nevertheless, changing the state of digital broadcast receiving apparatus always in accordance with a state of the radio wave causes a problem that it takes long time until the circuit becomes steady, and thereby, an image or a sound would be interrupted.
Furthermore, a digital broadcast receiving apparatus described in JP-A-2001-7713 is disclosed as follows.
A signal level of each analog broadcast channel is stored in advance as channel information in a channel information memory portion. In channel-selecting of a desired channel, an analog broadcast wave signal of a channel close to the desired channel is acquired from the memory portion. An operation starting point is set high to increase strength against a interference wave in the case that the interference wave is at a high level while the operation starting point is set low to cover a dynamic range in the case of no interference wave.
This enables, regardless of existence of a high interference wave having a high signal level in a close channel, the digital broadcast in a desired channel to be stably received while a low noise characteristic is kept.
The digital broadcast receiving apparatus to be used, which is built in a mobile terminal, is, however, also used as a mobile, so that a receiving point is changed as time passes. Therefore, the operation starting point should be also switched as time passes since a level of the interference wave also varies. This causes a problem that a signal is interrupted in switching the operation starting point.